1. Field of the Invention
The present invention relates to an ultrasonic welder having a motor drive assembly for positioning its ultrasonic horn.
2. Background Art
Ultrasonic welding joins plastic parts together by rapidly transmitting ultrasonic energy through a shaped tool known as an ultrasonic horn. The energy is in the form of mechanical vibrations and creates friction between the parts. The friction generates heat to melt and fuse (i.e., weld) the parts together.
An ultrasonic welder includes an ultrasonic transducer (i.e., an ultrasonic converter). The transducer is housed within a bottom portion of a housing of the welder. The horn is mounted to the bottom end of the transducer and is located adjacent to the bottom housing portion. The transducer is a piezoelectric element that mechanically vibrates upon receiving electricity. The vibration frequency typically falls within a range of 10-50 kHz. As the transducer vibrates at a given frequency, the horn incrementally moves up and down at the same frequency (for instance, on the order of±0.0004 inches per cycle).
In order to weld two parts together, the two parts are placed on top of one another. The horn is then brought into forced contact with the top part while the bottom part is supported on a stationary surface. As the transducer vibrates upon being supplied with electricity, the horn reciprocates vertically such that the horn resonates toward and away from the top part. The horn reciprocation in relation to the parts (i.e., the transfer of vibratory energy from the horn to the parts) creates the heat which fuses the parts together.
An ultrasonic welder typically includes either an air drive assembly or a motor drive assembly for positioning the horn relative to the parts (i.e., for moving the horn towards the parts to bring the horn into forced contact with the parts and for moving the horn away from the parts such that the horn is not in contact with the parts).
A typical air drive assembly includes an air cylinder having an actuator connected to the transducer housing. Compressed air forced into the cylinder causes the actuator to extend out from the cylinder to thereby move the transducer housing and the horn away from the cylinder until the horn contacts the top part. After the parts are welded together, compressed air forced into the cylinder causes the actuator to retract back into the cylinder to thereby move the transducer housing and the horn back toward the cylinder and away from the parts. Problems with air drive assemblies include the presence of an air source, precise movement of the actuator is not obtainable as compressed air is the mechanism which moves the actuator, and the air pressure which may realistically provided to the cylinder is relatively low resulting in forces applied by the horn onto the parts being relatively low and inconsistent.
A typical motor drive assembly includes a motor having a rotatable drive shaft connected to the exterior surface of the top face of the transducer housing. The motor is driven in response to an electrical signal to rotate the drive shaft one way causing the transducer housing and the horn to move away from the motor and toward the parts until the horn contacts the top part. After the parts are welded together, the motor is driven in response to another electrical signal to rotate the drive shaft the other way causing the transducer housing and the horn to move back toward the motor and away from the parts.
In addition to not using compressed air, advantages associated with a typical motor drive assembly include precise horn positioning as a result of electronically controlling the motor. Electronic motor control also provides indirect information regarding the positioning of the horn (for example, by monitoring the revolutions of the drive shaft and the like). A position transducer which directly monitors movement of the horn may be employed to provide information regarding the horn positioning.
However, a problem with an ultrasonic welder having a typical motor drive assembly is that drive components of the motor drive assembly are not integral with the transducer housing. For instance, the drive shaft is connected to the exterior surface of the top face of the transducer housing. Consequently, the overall size including the height of the ultrasonic welder is relatively large. Another problem with an ultrasonic welder having a typical motor drive assembly is that the drive components do not linearly guide the horn while positioning the horn. For instance, the drive shaft, as well as any other drive components such as guide shafts, are connected to the exterior surface of the top transducer housing face. Because the horn is mounted to the bottom transducer housing face, unintentional linear movement of the horn relative to the top transducer housing end (i.e., “wobbling”) can occur when the horn is in forced contact with the parts to be welded.